Loading on the components of wind turbines is carefully controlled, in order to maximise efficiency, and to minimise wear and damage. Load sensors are commonly mounted on wind turbines, typically on the rotor blades. These sensors require calibration in order to convert the values measured by these sensors to the actual loads on the turbine components.
One previously considered method for calibrating these sensors is a calibration procedure in which the rotor is turned while the blades are at a fixed pitch angle. Other previous methods fix the rotor, so that it is immobile while measurements relating to the blade load sensors are taken, for example while one blade is horizontal. Such approaches may not be sufficient to provide the data needed for more sophisticated models of the forces and moments acting on the blades. Such more sophisticated models may be required in order to avoid large inaccuracies in load estimation, which may lead to unsatisfactory wear or damage mitigation. It may also be practically challenging to stop the rotor at a specific position with exacting precision. In other previous methods, less sophisticated models are used for load calculations and calibration, such as neglecting the axial loads due to centrifugal effects and/or neglecting the influence of forces not along the measurement axis.
In some previous methods, the influence of axial loads can be, in ideal cases, addressed by using a differential sensor setup, in which two sensors are placed on opposite sides of the blade. These methods may not be sufficiently accurate where the sensors cannot reliably be placed exactly in alignment across the blade due to installation tolerance or due to the structural design of the blade. Less sophisticated models are further inadequate when three or more sensors per blade are used.
Another previous method rotates the rotor while other components of the turbine are also moved. However, in such methods, the testing procedure and/or movements must be heavily limited in order to prevent either stalling of the rotor, which requires a test re-start, or excessive speed of the rotor, which can damage turbine components. In addition, such methods are typically unable to test using a full range of movement of a turbine component without either stalling, producing excessive speed, or damaging the component or turbine.
In another previous method, a more sophisticated model for loading is used, but measurements are only taken during normal running of the turbine. This may not allow for full testing of all possible loading situations, and in any case again cannot test the full range of movement, for similar reasons as for the testing procedure above. Moreover, in normal running of the turbine all blades are operated at once.
The present invention aims to address these problems and provide improvements upon the known devices and methods.